If a : b = 20:5 then find out the value of (13a-9b): (11a-7b)?

The average of eleven numbers is 68. The average of the first numbers is 78 and that of the next four number is 63. The 9^th number is two time the 11^th number and the 10^th number is 4 less than the 11^th number. What is the average of the 9^th and 11^th numbers?

The ratio of ages of A and B 4 years ago was 5 : 3. The sum of present ages of A, B and C is 80 years. If present age of C is equal to sum of present ages of A and B. find the present age of A.

The average weight of 29 students is 40 kg. If the weight of teacher be included the average weight increased by 400 gms. The weight of the teacher is-

Simplification 

There are 100 students in 3 sections A, B and C of a class. The average marks of all the 3 sections was 84. The average of B and C was 87.5 and the average marks of A is 70. The number of students in A was. 

A person goes to his office by scooter at the speed of 80 km/hr and reaches 60 minutes earlier. If he goes at the speed 50 km/hr, then he reaches 48 minutes late. What will be the speed of the scooter to reach on the usual time?

If (2^x)(2^y) = 8 and (9^x)(3^y) = 81 , then ( x , y ) is :